The bushing system design for marine propellers has remained relatively unchanged since the early 1940's. Typically, a bushing is used to make a connection between the propeller body and the drive hub on the propeller shaft. The conventional bushing generally is formed from a hard rubber and makes the connection using friction. The rubber bushing is bonded to a center hub made of metal or plastic and the rubber is designed to be larger than the hole in which it is to be inserted. An installation funnel is used to compress the diameter of the rubber bushing to enable it to be inserted into the propeller bore. One of the benefits of this conventional design is that it gives both impact protection and harmonic vibration absorption protection. Another benefit of this conventional design is that, under impact, the rubber bushing will slip and, in most cases, reconnect or lock up again and enable the boat to continue to drive, at least under limited power.
When the maximum horsepower of boats with outboard motors was 25 hp, this conventional bushing design was ample. However, current horsepower ratings of boats far exceed the design capabilities of such a conventional bushing. With so much torque under impact, the rubber bushing slips and melts. As a result, the bushing does not return to its normal size and becomes unusable in seconds. The damaged conventional bushing remains loose within the propeller bore, leaving the boat drive system useless and the boat undriveable.
To correct this problem with higher horsepower motors, manufacturers have placed a hard plastic or metal keyed piece to operatively engage the hub of the propeller. While this method reduces failures akin to the ones mentioned above with the conventional rubber bushings, it does not provide any protection for the drive train under impact, it does not absorb any harmonic vibration from the motor or drive train, and it does not remain sufficiently tight on the propeller shaft. The latter issue induces a rattle in the propeller shaft and produces operating noise. It also promotes wear and tear on all the drive components.
There are other designs that have the same and other pitfalls as mentioned herein above. Therefore, what is needed is a propeller bushing that provides the protection of a rubber bushing, while providing the positive lock of a keyed system for higher horsepower motors.